1. Field of the Invention
The present invention generally relates to a mobile wireless communication system, an access gateway, a wireless base station, and a mobile wireless control method.
2. Description of the Related Art
Currently, various wireless communication systems, which include plural wireless base stations connecting with upper access gateways via a network and plural mobile stations communicating on the move or in a stationary state via the wireless base stations, are proposed and put to practical use. One example is a next generation mobile wireless communication system referred to as the LTE (Long Term Evolution) system. The LTE system includes a MBMS (Multimedia Broadcast Multicast Service) which defines a multicast service and a broadcast service for providing the same data to plural mobile wireless stations located at a predetermined area. The multicast service is for providing services to users that have registered and joined a predetermined multicast service. The broadcast service is for providing services to all users located in a predetermined service area. In the broadcast service, the user can decide whether to receive the data of the service and requires no exchange control signals with respect to the network side (See, for example, 3GPP TR25.814 v7.0.0 (2006-06) (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA) 7.1.1 Basic transmission scheme).
As another mobile wireless communication system providing a multicast service and a broadcast service, there is proposed a system including a wireless base station(s) and an upper control station(s). This system employs logic channels including MTCH (MBMS point-to-multipoint Traffic Channel) for transmitting data and MCCH (MBMS point-to-multi-point Control Channel) for transmitting control data and a physical channel including MICH (MBMS Notification Indicator Channel) for notifying stations whether control data are being transmitted. In this system, the control station is able to change the time for repetitively transmitting the same control data and sends data indicating the timing of the transmission to the wireless base station. Accordingly, the wireless base station can be notified of the timing change, wherein the timing corresponds to a value no less than a value obtained by adding the difference between a first count value of a counter of the control station and a second count value of a counter of the wireless base station to the first count value.
In operating with the above-described MBMS (MBMS (Multimedia Broadcast Multicast Service), it is necessary for plural wireless base stations to transmit the same data at the same timing to one or more wireless mobile stations. In other words, in a case where plural wireless base stations cannot transmit the same data at the same timing, the wireless mobile station cannot attain continuity of data before and after the handover process. Therefore, the plural wireless base stations are to be synchronized with each other. In the above-described Non-patent document 1, the range of guard-interval (Cyclic Prefix: CP) is defined as a Long CP (Cyclic Prefix) Length of 16.67 μsec. That is, in a case of synchronizing among the plural wireless base stations, a precision of a μsec order is required. In order to maintain such time precision, a high precision oscillator is to be provided in each wireless base station. Furthermore, in operating the MBMS, it is necessary for an access gateway to transmit the same message to each wireless base station and for each the wireless base stations to transmit data to the wireless mobile station at the same time while still maintaining such time precision.